This invention relates to methods of operating pneumatic impact tools, particularly to self-propelled ground piercing tools. Wentworth U.S. Pat. No. 5,505,270, Apr. 9, 1996, the entire contents of which are incorporated by reference herein for all purposes, describes a reversible pneumatic ground piercing tool having a reversing mechanism with a supplemental air line capable of supplying compressed air for reverse operation to a radial port in the air distributing mechanism. This radial port is located between a pair of bearing surfaces on the step of the air inlet conduit, and when pressurized by the supplemental air line, causes the front pressure chamber to receive compressed air earlier than normal, shifting the stroke of the striker rearwardly so that the tool operates in reverse. Opening the supplemental air line to the atmosphere produces a short stroke forward mode of operation useful for operations wherein a less forceful impact is desirable.
Experience with the tool of the '270 patent revealed areas for possible improvement. The short stroke forward mode finds little practical application, and thus it is not essential to provide for it. The reverse impact of the '270 tool is relatively weak and not sufficient for use in vertical applications such as pile driving. Problems were also encountered with double-hitting, where the striker during reverse mode hits against both the front and rear anvil surfaces rather than stopping short of the anvil or front impact surface. The present invention addresses these difficulties.